1. Field of the Invention
The present invention relates to a battery state monitoring circuit for controlling charge/discharge of a plurality of secondary batteries, and a battery device including the battery state monitoring circuit.
2. Description of the Related Art
FIG. 4 illustrates a circuit diagram of a conventional battery device. In the conventional battery device, two batteries 301 and 302 as secondary batteries are interposed in series between power supply terminals +VB and −VB of a battery state monitoring circuit. A connection point between the two batteries is connected to a VI terminal of the battery state monitoring circuit. A voltage of the battery 301 is divided by a voltage dividing circuit 304. The divided voltage is detected by a voltage detection circuit 305. An output of the voltage detection circuit 305 is input to a control circuit 308. If any one of the batteries is in an overcharged state or an overdischarged state, the control circuit 308 outputs a signal Vs for turning OFF a switch (not shown) provided between the secondary batteries and an external power supply terminal. The control circuit 308 is therefore constituted by a logic circuit alone. Similarly, it is detected by a voltage dividing circuit 306 and a voltage detection circuit 307 whether or not the battery 302 is in an overcharged state or an overdischarged state. A result of the detection is input similarly to the control circuit 308 as a digital signal. Therefore, if any one of the batteries 301 and 302 becomes the overcharged state or the overdischarged state, the control circuit 308 disconnects the batteries and the external power supply, thereby being capable of stopping the progression of overcharge or overdischarge. Two batteries do not have exactly the same charging characteristics and discharging characteristics, and hence it is necessary to detect and control the overcharge and the overdischarge on a battery basis (see, for example, Japanese Patent Application Laid-open No. Hei 08-308115).
The conventional technology, however, has a problem that, if only the secondary battery 301 becomes the overcharged state or the overdischarged state, power of only the secondary battery 301 is consumed by the voltage detection circuit 305 to result in unbalanced voltages between the secondary batteries. If the secondary batteries are charged under the unbalanced voltage state, the charge is stopped when the secondary battery with the highest voltage becomes the overcharged state even if the other batteries have not been sufficiently charged. On the other hand, if the secondary batteries are discharged under the unbalanced voltage state, the discharge is stopped when the secondary battery with the lowest voltage becomes the overdischarged state even if the other batteries still have high voltages. Therefore, there is a problem that the life of the battery device is shortened.